This invention relates to the production of highpurity carbon by the purification of coal.
In recent years, much attention has been given to the purification of coal for metallurgical or environmental reasons. Simpler schemes for removing a majority of the impurities involve beneficiation processes such as a flotation process which may reduce the impurity level down to about 1-3%. Literature sources indicate that at least experimental work has been carried out toward further reduction of the impurities by a caustic digest followed by an acid treatment. [The Preparation of Ultra-Clean Coal in Germany, by A. Crawford, Transactions -- The Institute of Mining Engineers, Volume 111, pages 204-19 (1951-1952); The Action of Alkalies on Low-Rank Coals, by J. D. Brooks and S. Sternhell, Fuel, Volume 37, pages 124-125 (1958); Coal as a Source of Electrode Carbon in Aluminum Production, by R. J. Campbell, Jr., R. J. Fulton and C. L. Boyd, Bureau of Mines Reported Investigations 5191, (1956); Preparation of Ash-Free, Pyrite-Free Coal by Mild Chemical Treatment, by L. Reggel, R. Raymond, I. Wender and B. D. Blaustein, ASC, Division of Fuel Preprints, Volume 17 (1), pages 44-48 (1972)] While in each of the above articles a coal is produced which is considerably purer than the starting material, none of the references reports production of a purified material containing less than 0.1% ash. Furthermore, the reduction of other undesirable impurities, such as sulfur, to levels below 1.5% total sulfur and preferably substantial elimination of pyritic sulfur (sulfides of iron) is also desired but apparently unobtainable using prior art processes such as described, for example, in the aforesaid Reggel et al article. Such a high level of purity is required for example for use as carbon aggregate in aluminum smelting anodes.
In the article by Crawford the coal was subjected to a 3% caustic digest at 250.degree. C. for 20 minutes followed by a 5% HCL treatment to reduce the ash content from 0.8 to 0.28%. Brooks et al subject the coal to a caustic treatment at 190.degree. C. for 12 hours followed by unidentified acidification. A level of 0.1% is reported for one particular sample of coal. Other samples are reported at 0.3% final ash content. Campbell et al report that a 0.2 content ash level was achieved by subjecting a beneficiated coal to a combination sodium hydroxide and butyl alcohol digest at 125.degree. C. for 2 hours followed by an acid treatment in either hydrochloric acid, sulfuric acid, CO.sub.2 or SO.sub.2. The ash content was reported to be reduced to 0.7%.
While the foregoing treatments undoubtedly represent progress in the art toward lowering the content of impurities such as, for example, sulfur, to acceptable environmental levels for combustion purposes and even for some metallurgical purposes, there still remains a need for a process to produce an even higher purity carbon for use, for example, in aluminum smelting anodes. It is therefore an object of this invention to provide a treatment for the purification of coal which will lower the ash content of the coal to less than 0.1%.